Syncing data and images with a client in a web application

I'm writing a web application in PHP which need开发者_高级运维s to store images and image meta data. In future, the application may need to work offline on the client. A user might need to download all the images and data to his laptop before going to a remote area without internet access. Whilst at the remote location the user could add new images to the system and be able to compare them with his local copy of the image database. When returning to an area with internet access, the user would run a sync operation which would copy his new images to the server and retrieve any new ones.

I've looked at the new web storage / localstorage options in HTML5 (web sql database seems to have been dropped) and I think this is going to be too limited as there is only 5MB space and one or two images could easily exceed that.

Is what I want to do actually possible / practical with a browser-based web application? Or should I be looking at writing a desktop/tablet application with local file storage capabilities for users without net access. Initially, it does need to be a web application, I'm just trying to think ahead. Will I give myself more options in future by using something like couchDB for the backend from the start? As I understand it, this comes with good syncing functionality.

Thanks,

I decided to use Titanium Desktop.

http://www.appcelerator.com/products/titanium-desktop-application-development/